1. Field of the Invention
The present invention is in the field of pretreating aluminum or aluminum alloy surfaces prior to diffusion bonding by means of a chemical etchant.
2. DESCRIPTION OF THE PRIOR ART
It is well known that high strength and high modulus materials can be formed from boron fiber reinforced aluminum sheets, which make them attractive materials for aerospace applications. Much of the technology and material application have been directed toward structural applications. In view of the impressive specific properties of these composites, however, they have also been used for fan and compressor blade applications in turbojet engines. The fabrication of such composites into turbine hardware has been extensively developed and the following U.S. Pat. Nos. are merely cited by way of background:
Gray et al., 3,600,103; Athey et al. 3,711,936; Kreider 3,606,667; Stone 3,731,360; Alexander 3,649,425; Alver et al. 3,749,518; Kreider 3,699,623; Carlson et al. 3,762,835; Stone 3,701,190; Whitaker 3,942,231.
A boron-aluminum composite consists essentially of continuous boron fibers arranged in a uniform array to provide optimum reinforcement in an aluminum matrix. Fabrication methods for composites of this type may include;
(1) solid state processes, PA1 (2) liquid metal processes, and PA1 (3) deposition processes.
The present invention is directed to composites which are fomred by means of solid state, hot diffusion bonding. In this type of process, boron filaments are wound into filament mats which are subsequently disposed between aluminum foils and consolidated under heat and pressure in either an environment of air or under vacuum. During consolidation, a metallic bond is formed between the maxtrix foils by solid state diffusion.
One problem which has hindered the successful flight testing and subsequent application of composite blades of this type in gas turbine engines has been their inadequate impact resistance, resulting in reduced resistance to foreign object damage. A composite material used in boron-aluminum fan blades which has failed the foreign object damage test was charcterized by notched Charpy impact values of less than 10 foot-pounds as compared with a typical 15 to 20 foot-pounds impact value for conventional blade material consisting of titanium-aluminum-vanadium alloys.
The Charpy impact energy in boron-aluminum composites is strongly influenced by the matrix composition and fabrication procedures which affect the strength level of the fiber-matrix bond. To achieve a maximum impact resistance in boron-aluminum, it has been found necessary to have a strong bond at the matrix-matrix interface and a moderate-to-weak bond at the fiber-matrix interface. To achieve the latter, low temperature and short bonding times have been used. However, at such low temperature-time combinations, it is difficult to achieve a sufficiently strong matrix to matrix bond.